Molecular Film on Liquid Mercury Reveals New Properties

UPTON, NY -- A team of scientists from the U.S. Department of Energy's
Brookhaven National Laboratory, Harvard University, and Bar-Ilan
University in Israel have grown ultrathin films made of organic molecules
on the surface of liquid mercury. The results, reported in the November
15, 2002, issue of Science, reveal a series of new molecular
structures that could lead to novel applications in nanotechnology, which
involves manipulating materials at the atomic scale.

Growing molecular films on liquid surfaces is part of an ongoing
activity by Brookhaven scientists to create nanomaterials, which are a few
billionths of a meter in thickness. Ultrathin films are becoming
increasingly important for fast-developing applications, such as faster
and smaller electronic and magnetic devices, advanced biotechnological
membranes, and controlled drug release in the human body. The Brookhaven
team is a leader in the field of liquid surface-supported film growth,
with expertise gained over the past 20 years.

"When you
grow a film on a solid surface, the molecules of the film tend to
interlock with those of the underlying support," says Benjamin Ocko, the
Brookhaven physicist who participated in the study. "But an underlying
liquid surface is not ordered and provides an ideal setting for studying
ultrathin states of matter without the complications of the solid
support."

Ocko and his colleagues first filled a small tray with liquid mercury
and then deposited on the surface a nanometer-thin film of stearic acid,
an organic waxlike material that is a common component of cell membranes.
Since stearic acid is not soluble in mercury, it floats on the surface.

To see how the molecules of the film organize on the surface, the
scientists measured how x-rays produced by the
National
Synchrotron Light Source at Brookhaven scattered off the ultrathin
molecular film. Key to the study was a unique instrument used for tilting
the x-rays downward onto the liquid mercury surface, which was developed
by Peter Pershan, a physicist at Harvard and one of the study’s authors,
along with the Brookhaven team.

The scientists discovered that, as the number of molecules deposited on
the surface increased, they formed four distinct patterns. "First, when a
few molecules are deposited, they tend to take as much space as they can,
by lying on the surface," explains Henning Kraack, a physics Ph.D. student
from Bar-Ilan and the study's lead author. "When more molecules are added,
a second layer of molecules lies on top of the first one.

This schematic drawing shows how the stearic
molecules of the film rearrange as they are added onto the surface
of the liquid mercury support.

"Then, as even more molecules are deposited," Kraack continues, "they
'stand up' to leave more space to neighboring molecules, allowing them to
densely pack in one layer. But even then, before standing up straight, the
molecules are first tilted to the side, and stand up completely only when
they are 'squeezed' by other molecules that 'elbow their way through.'"

These observations came as a surprise, since previous studies have
shown that, when stearic molecules are deposited on water -- the only
other liquid support studied so far -- they only stand up on the surface.
"Patterns in which molecules lie flat on a liquid surface have never been
observed before," Kraack says.

Moshe Deutsch, a physicist at Bar-Ilan and one of the authors of the
study, notes that because the liquid mercury does not seem to influence
too much the way the stearic molecules assemble, "growing films on a
liquid surface is like growing them without support at all." It might be
possible to choose a film pattern, he adds, simply by selecting the
appropriate molecular coverage.

"This work shows that without an underlying lattice, we can control
film growth," Deutsch says. "By growing other molecules on a liquid
support, we will be able to control the size and properties of other
films, and thus tailor them for different applications, in particular
their use in nanoelectronics and nanosensor technology."

This work was funded by the U.S. Department of Energy, which supports
basic research in a variety of scientific fields, the National Science
Foundation, and the U.S.-Israel Binational Science Foundation in
Jerusalem, Israel.

The
U.S. Department of Energy's Brookhaven National Laboratory conducts
research in the physical, biomedical, and environmental sciences, as
well as in energy technologies. Brookhaven also builds and operates
major facilities available to university, industrial, and government
scientists. The Laboratory is managed by Brookhaven Science
Associates, a limited liability company founded by Stony Brook
University and Battelle, a nonprofit applied science and technology
organization.